Just buy a solid wood door and a dead-bolt lock - that will be enough to keep kids out, and even workmen while you are present (and probably if you aren't present.) In fact, that's what is typically used in Condos that are used as part-time rentals to keep "honest" folks out of the owner's stuff.

Nothing mentioned above is going to keep a thief out who is determined to get at your guns. I could cut through all of the above with nothing more than a $22 sawzall from Harbor Freight.